Consider a state that is an admixture of two neutrino species,
\(|\nu _e\rangle \)  and \(|\nu _a\rangle \)  or, equivalently, \(|\nu _1\rangle \) 
and \(|\nu _2\rangle \) . With some approximations, the time evolution
can be written in the following matrix form [1]}:
\(-i\frac{\partial }{\partial x} \begin{pmatrix}\nu _e\\\nu _a\end{pmatrix}=-\frac{1}{2E}M_w^2 \begin{pmatrix}\nu _e\\\nu _a\end{pmatrix},\) 
